C sp2 -H Amination Reactions Mediated by Metastable Pseudo- O h Masked Aryl-Co III -nitrene Species.

Cobalt-catalyzed C-H amination via M-nitrenoid species is spiking the interest of the research community. Understanding this process at a molecular level is a challenging task, and here we report a well-defined macrocyclic system featuring a pseudo- O h aryl-Co III species that reacts with aliphatic azides to effect intramolecular C sp2 -N bond formation. Strikingly, a putative aryl-Co═NR nitrenoid intermediate species is formed and is rapidly trapped by a carboxylate ligand to form a carboxylate masked-nitrene, which functions as a shortcut to stabilize and guide the reaction to productive intramolecular C sp2 -N bond formation. On one hand, several intermediate species featuring the C sp2 -N bond formed have been isolated and structurally characterized, and the essential role of the carboxylate ligand has been proven. Complementarily, a thorough density functional theory study of the C sp2 -N bond formation mechanism explains at the molecular level the key role of the carboxylate-masked nitrene species, which is essential to tame the metastability of the putative aryl-Co III ═NR nitrene species to effectively yield the C sp2 -N products. The solid molecular mechanistic scheme determined for the C sp2 -N bond forming reaction is fully supported by both experimental and computation complementary studies.